ARC-CO payments are made based upon the Olympic Average yield for each county on a covered crop-by-crop basis. Normally, all of the production is totaled for the county and then divided by planted acres to arrive at the actual yield for that county.
However, if enough of the production in that county for that covered crop is irrigated (usually 25%), then there are separate calculations for irrigated and non-irrigated. In some cases, you would prefer for there to be separate calculations, other times, you may wish for there not to be one. Normally, a county with some irrigation, (but not to the level of 25% or more) will increase the overall yield for the county which will result in greater potential payments to the producer. However, if there is a separate yield for both, then the dry land grower will have a potentially lower payment and the irrigated farmer will have higher, but will not have as much variability in yields.
The FSA has a spreadsheet on their site (the 8th one down on the far left list) that shows all of the updated counties in the US with both irrigated and non-irrigated yields. You can check the spreadsheet and see if your county is listed.